1. Field of the Invention
The present invention relates to a laser scanner for which the light source is comprised of a semiconductor laser. In particular, it relates to a circuit for detecting a scanning sync signal that is used for controlling the timing of a laser beam scanning.
2. Description of the Related Art
In a laser scanner which utilizes a semiconductor laser and so forth for a light source, a required graphic pattern is transferred to a photosensitive member by driving the photosensitive member in a sub scanning (vertical scanning) direction while a main scanning (horizontal scanning) is carried out for the photosensitive member by the laser beam. In this type of the laser scanner, the horizontal scanning of the laser beam should be synchronized with the vertical scanning motion of the photosensitive member. Therefore, a detection of a horizontal sync signal is required.
For example, FIG. 7 illustrates a general construction of a laser scanner 1. A laser beam LB, emitted from a semiconductor laser 11, is collimated by a collimator lens 13 and shaped through a cylindrical lens 14, then projected to a polygon mirror 15 which is rotating at high speed. The laser beam LB is then reflected toward a direction of the main scanning by a reflecting mirror of the polygon mirror 15, and the main scanning at a photosensitive surface of a photosensitive drum is carried out via an fxcex8 lens 16. The photosensitive drum 17 has the rotating axis 17a which is parallel with the main scanning direction. The drum is rotated around the rotating axis 17a, so that the photosensitive drum is sub scanned by the laser beam LB and a required pattern is formed on the drum surface.
Further, a laser detector 12, comprised of a photodiode, is disposed at a side of the photosensitive drum 17. The detector 12 receives the laser beam LB just before the beginning of each main scanning and sends detecting signals to a sync-detecting circuit 2. From the detecting signals, the sync-detecting circuit 2 generates a horizontal synchronizing signal HSYNC which standardizes the timing of the main scanning. Proper horizontal scanning with the laser beam LB is achieved by controlling the speed and timing of the polygon mirror""s 15 rotation, so that the horizontal synchronizing signals are generated in a predetermined timing.
In general, as for the above-mentioned laser detector 12, a package-made detector is used. As shown in FIG. 8, a photodetector 32 is provided in the package. The photodetector 32 is disposed on a photodetector circuit board 31 and is electrically connected to the circuit patterns 34 of the circuit board 31 with bonding wires (jumper wires) 33, and entirely enclosed with a transparent resin 35.
Laser beam scans over the laser detector 12 are in the order A, B, C, and D shown in the figure. However, in the case of the type of laser detector 12, laser beams, such as B, C, or D, which penetrate the transparent resin 35 before and after the proper beam to be detected for generating the horizontal synchronizing signal, which incidents straight on the photodetector 32, are reflected by the circuit patterns 34 of the photodetector circuit board 31 and bonding wires 33 and the like, or by the inner surface of the transparent resin 35. The reflected laser beams are then made incident to the photodetector 32 as stray light. Therefore, in some cases, the laser detector 12 misidentifies these stray lights as a horizontal synchronizing signal. As a result, when the misdetection of a horizontal synchronizing signal occurs, the laser scanner 1 fails in transferring a proper image.
In the case of the laser detector shown in FIG. 8, in order to avoid the above misdetection of the horizontal synchronizing signals, a stray-beam-preventing slit board 37 may be provided to reduce the stray beams made incident on the photodetector 32 by allowing only the proper beam to penetrate the slit. However, it is difficult to prevent the generation of the stray beams originating from the proper beam, the beams which penetrate the slit board 37, and light reflected inside the laser detector as described above.
Further, although the antireflection coating (not shown) for preventing the reflection inside laser detector may be applied for structural improvement, it is still difficult to completely prevent the reflection.
Furthermore, from the aspect of an electrical circuit, a sync-detecting circuit can be designed to reject a detecting signal arising during a period when no synchronizing signal should possibly be detected. However, it is hard to distinguish and reject a detecting signal generated by the stray beams from the proper signal when the detecting signal arises at a time close to the time of the proper beam to be detected. As a result, the misdetection of the synchronizing signal induced by the stray beams remains.
Since the stray beams are reflected several times before they reach the photodetector, the power of each stray beam is attenuated and is lower than that of the proper beam. Thus the stray beams may be distinguished and rejected by setting a slice or boundary level of the slice circuit for output of the laser detector.
FIG. 9 illustrates an example of the slice circuit of the sync-detecting circuit to which the boundary level is applied. Waveforms illustrated in FIG. 10A represent normal behavior of the sync-detecting circuit. A photocurrent produced in the laser detector 12 by the effect of incident laser beam LB is transferred to a voltage output Va at an I-V converter 301, and then applied to the noninverting input terminal of a comparator 302. A standard voltage V1 that is higher than the voltage normally produced by the stray beams is applied to the inverting input terminal of the comparator 302 from a criterion power source 303. The output voltage Va and the standard voltage V1 are compared at the comparator 302. When the output Va is higher than the standard voltage V1, a high level output Vb is output from the comparator 302 and the horizontal synchronizing signal HSYNC is output from a driver 304 in accordance with the output Vb. Namely, outputs of the horizontal synchronizing signals can be synchronized with the laser beam LB reception at the laser detector 12 by classifying the output signal Va as to the boundary level which is set to the standard voltage V1.
However, in the case of a laser scanner for which the power of the proper beams are varyingly controlled, such as a laser scanner that controls the power of the laser in accordance with the sensitivity of the photosensitive member or the depth of the image to be formed, a stray beam may be detected as the synchronizing signal.
For example, as shown in FIG. 10A, when the powers of the stray beams are sufficiently small compared to the power of the proper beam, the proper beam can be discriminated from the stray beams by setting the boundary level or the standard voltage appropriately, so that the synchronizing signals are detected properly, as described above. However, as shown in FIG. 10B, when the power of the laser beam is increased and the output Va of the proper beam exceeds a saturating level of the photodetector, the power of the stray beam is increased as well. So that, in this case, the output Va of the stray beam may exceed the boundary level V1. As a result, the comparator 302 may output Vb2 by the stray beam as well as output Vb1 by the proper beam. Thus, the driver 304 may output the synchronizing signal HSYNC in error, in accordance with the output Vb2 by the stray beam.
Further, the power level of the stray laser beam, in the high-power mode, may become higher than the level of the proper laser beam in the low power mode depending on the setting of the laser power controls. In this case, if the standard voltage of the boundary level is set at a certain constant voltage, the stray beam may be detected as the synchronizing signal.
Therefore, an object of the present invention is to provide a synchronizing signal detecting circuit for a laser scanner that prevents misdetections of a synchronizing signal by a stray beam and enables a precise detection of the synchronizing signal, independent of the laser beam power.
According to the present invention, a scanning sync-signal detecting circuit is provided that is utilized in a laser scanner that comprises a laser detector for detecting a laser beam to be scanned and a sync-detecting circuit that processes an output signal from the laser detector so as to output a scanning sync signal.
The sync-detecting circuit detects a proper beam for generating the scanning sync signal in accordance with the duration of the output signal of which the level is higher than a predetermined level. The sync-detecting circuit then generates the scanning sync signal in accordance with the proper beam.
From another aspect of the present invention, a laser scanner is provided that utilizes the above sync-detecting circuit.
For example, the sync-detecting circuit comprises a ramp generator, a comparator, and a sync signal generating processor. The ramp generator generates a ramp signal of which the level corresponds to the duration of the output signal from the laser detector. The comparator compares the level of the ramp signal with a predetermined boundary level, so that the output signal due to the proper beam is detected. The sync signal generating processor generates the scanning sync signal depending on the output of the comparator.
In the above example, the sync-detecting circuit is preferable to comprise a pre-comparator that compares the output signal from the laser detector with a predetermined pre-boundary level. The pre-comparator feeds the output signal to the ramp generator while said output signal from the laser detector is above or equal to the pre-boundary level, so that the ramp signal is generated.
Further it is also preferable that the ramp generator integrates signals at a predetermined level while the output signal from the pre-comparator is input to the ramp generator, so that the ramp signal is generated.
In another example, the sync-detecting circuit comprises a clock, a counter, a count-number comparator, and a sync signal generating processor. The clock generates regular clock pulses and the counter counts the clock pulses while the output signal from the laser detector is input to the counter. The count-number comparator compares a count number counted at the counter with a predetermined criterion number, so that an output signal from the laser detector, which corresponds to the count number above or equal to the criterion number, is detected as a signal due to the proper beam. The sync signal generating processor generates the scanning sync signal in accordance with an output signal from the count-number comparator.
In the above example, the sync-detecting circuit is preferable to comprise a pre-comparator that compares the output signal from the laser detector with a predetermined pre-boundary level. The pre-comparator feeds the output signal to the counter, while the output signal from the laser detector is above or equal to the pre-boundary level, so that the count number is obtained.
Further preferably, the counter counts the clock pulses while the output signal from the pre-comparator is input to the counter. The count number of the continuous clock pulses is only output from the pre-comparator.
Furthermore, the laser scanner preferably comprises a horizontal scanning processor and a vertical scanning processor. The horizontal scanning processor scans the laser beam in a horizontal direction and the vertical scanning processor scans the horizontal laser beam in a direction vertical to the horizontal direction. Moreover, the laser detector detects the laser beam being horizontally scanned and the sync-detecting circuit detects a horizontal synchronizing signal.
An example of the laser detector is a package-made detector. The detector comprises a photodetector and outputs a detecting signal when the photodetector receives the laser beam on a receiving surface.